1. School of Engineering, Design and Technology, University of Bradford, Bradford BD7 1DP, U.K.
2. Institut Laue Langevin, 6 rue Jules Horowitz, BP 156 38042 Grenoble Cedex 9, France
3. School of Mathematical Sciences, University of Nottingham, Nottingham NG7 2RD, U.K.
4. Departments of Physics and Chemistry, Durham University, Durham DH1 3LE, U.K.